For reception of television and other broadcast signals, it is usually preferable to have an antenna with some directivity to discriminate undesirable multipath signals. Such a directional antenna can be rotated mechanically toward a broadcast transmitter. However, the mechanical rotation is undesirable since it requires additional costly rotating equipment. Another way of directing the antenna is to select one of the fixed antennas, which is originally oriented toward a desired transmitter. The notion of switching between different antennas is well described in S. Takeda et al. "FM Mutlipath Distortion in Automobile Receivers Has Been Significantly Reduced by A New Antenna System" pp. 263-269 IEEE Transaction on Consumer Electronics, Vol. CE-26, August 1980. More specifically, this article discloses a switching antenna system including the two loop antennas orthgonally combined. Once multipath interference occurs for one loop antenna being used, the multipath distortion sensor will detect it and a user can switch for the alternative.
As for antenna switching circuits, many arrangement have been proposed. One of such arrangements is described in U.S. Pat. No. 4,959,873 granted to Flynn et al. In Flynn et al., a signal output line is switched to one of two or more input lines to which it is permanently connected at a common junction. Each input line has an associated amplifier stage which can be biased in a normal high gain ("on") state, or in an isolation ("off") state. Biasing in the "off" state causes that the amplifier stage output presents a low impedance to its own input line, the length of which is chose to reflect a high impedance at the junction with the other lines. This switching arrangement enables the return loss and insertion loss of the "on" path to be kept to low values while simultaneously offering a high insertion loss to the "off" path signals. However, this arrangement is undesirable especially for switching VHF/UHF signals since it requires a particular length of the input line which could significantly limit design freedom for the circuitry. Therefore, a need exists for an RF switching arrangement that provides acceptable isolation for the "off" signals in VHF/UHF bands of frequencies without demanding an additional attenuation circuit.